This invention relates to determining the concentration of water soluble species in a biological fluid.
Sodium and potassium ion concentrations in blood plasma, for example, can be measured directly (by direct potentiometry using an ion selective electrode) or indirectly (by flame photometry or indirect potentiometry procedures, which involve sample dilution). Methods which involve diluting the sample, however, tend to underestimate the concentration of water soluble species because the plasma water fraction of the sample (which includes the water soluble species and represents only a portion of the whole sample) is effectively diluted more than are the separate, purely aqueous, calibrating solutions used in the measurement. The effect increases with increasing protein or lipid concentration, e.g.. in pathological samples (Shyr et al., Clin. Chem. 26:1517 (1980); Coleman et al., Clin. Chem. 27; 1938-1939 (1981)).